1. Field of the Invention
The invention relates to a process and device for the ionic analysis of an insulating sample.
Conventionally, in spectroscopy, a target at the surface of the sample to be analyzed is bombarded by a beam of primary ions. The target then emits negative ions which are collected by a system which gives the distribution pattern of the elements present at the surface of the sample.
This technique is satisfactory for samples which are sufficiently conducting.
But, a large number of samples examined by the ionic analyser are insulating: polished rock sections, sections of teeth or bones, sections of biological tissues, oxide inclusions, oxide layers, passivation layers . . . Observation of these samples causes problems because the electric charges flow badly. Even when a balance is reached between the flows of charges received, emitted and evacuated by low conductivity, charge excesses or defects may exist locally and create potentials which stagger the energy distributions of the secondary ions, deform the paths of the secondary ions which will form the images and modifiy the primary bombardment conditions. Furthermore, the electric fields may be locally very intense and induce the migration of a number of elements. This phenomenon is particularly visible when implantation or diffusion profiles in silica layers on semi-conductors are plotted.
2. Description of the Prior Art
This problem has been partially solved in the case where, for removing the positive ions emitted by the target, a metal grid is deposited by evaporation on the surface. The insulating areas of the sample received, in addition to the primary ion beam, low energy secondary electrons emitted by the bars of the grid and high energy electrons produced on the extraction electrode and attracted by the sample. Control of the surface potential of the sample is then obtained by suitably adjusting the density of the primary bombardment.
On the other hand, when it is a question of negative secondary ions, on the one hand the electrons produced on the extraction electrode are pushed back by the sample and, on the other hand, the electrostatic field extracts at the same time the secondary electrons emitted by the target: the emission of secondary electrons is much more intense then the ionic emission, so that, whatever the sign of the primary bombardment ions, a positive charge always appears at the surface. This emission is further increased when the output work is lowered by a Cs.sup.+, K.sup.+ bombardment or by Cs vapor blowing. Moreover on heterogeneous samples, this charge may vary from one place to another depending on the secondary electron output of the location considered. Experience shows that this positive charge is such that any ionic microscopy from negative secondary ions is impossible with conventional procedures.